The detection and analysis of individual particles or cells is important in medical and biological research. It is particularly important to be able to measure characteristics of particles such as concentration, number, viability, identification and size. Individual particles or cells as herein defined include, for example, bacteria, viruses, DNA fragments, cells, molecules and constituents of whole blood.
Typically, such characteristics of particles are measured using flow cytometers. In flow cytometers, particles which are either intrinsically fluorescent or are labeled with a fluorescent marker or label, are hydrodynamically focused within a sheath fluid and caused to flow past a beam of radiant energy which excites the particles or labels to cause generation of fluorescent light. One or more photodetectors detect the fluorescent light emitted by the particles or labels at selected wavelengths as they flow through the light beam, and generates output signals representative of the particles. In most cytometers, a photodetector is also used to measure forward scatter of the light to generate signals indicative of the presence and size of all of the particles.
U.S. Pat. No. 5,547,849 describes a scanning imaging cytometer wherein an unprocessed biological fluid sample is reacted with a fluorescently labeled binding agent. The reacted sample undergoes minimal processing before it is enclosed in a capillary tube of predetermined size. The capillary tube with the enclosed sample is optically scanned and the fluorescent excitation is recorded from a plurality of columnar regions along the capillary tube. Each columnar region is generally defined by the spot size of the excitation beam and the depth dimension of the capillary tube. A spacial filter of sufficient pinhole diameter is selected to allow simultaneous volumetric detection of all fluorescent targets in each columnar region. The cellular components or particles are identified as is their concentration.